Nitrogen+Syngas 401 May-Jun 2026
27 May 2026
Plant Manager+
Problem No. 77 How airflow symmetry drives dust in urea prilling towers
In an ideal natural draft urea prilling tower, molten droplets fall, cool air rises, and prills solidify like raindrops in motion – simple physics, elegant design. But what happens when airflow at the tower base is disturbed? Small adjustments can have big consequences.
In some plants, operators place rubber sheets or old conveyor belts over parts of the air inlet grating to prevent prill drift. At first glance, this seems harmless; after all, the inlet isn’t fully blocked. However, fluid dynamics behaves differently: blocked sections create local high-velocity jets, turbulence, and cross currents, leading to:
- prills drifting sideways;
- prill to prill collisions;
- contact with tower walls or hopper surfaces;
- fines generation.
Ironically, an intervention meant to reduce drift can increase turbulence and dust.
Non-uniform damper adjustments
Operators often adjust dampers in response to wind direction. When settings are asymmetrical, droplet trajectories on the high velocity side are disrupted and collisions increase. On the low velocity side, prills cool more slowly and reach the bottom warmer and softer. The result? Dust generation at the tower discharge, even when prill quality and crushing strength are acceptable.
Other overlooked effects include: local air acceleration – partial inlet blockage increases velocity, lifting light prills and increasing attrition; and ambient wind – swirling flow causes prills to drift toward walls, prompting more adjustments and turbulence.
Dust generation is therefore often more about airflow stability than prill melt quality.
How experienced plants handle it
Rather than blocking inlets, many plants focus on:
- maintaining uniform annular inlet area;
- keeping damper settings symmetrical;
- installing wind shields at tower base;
- controlling droplet size distribution.
Uniform airflow is far more important than airflow reduction in maintaining prill integrity.
Research and industry insights
CFD modelling confirms that airflow patterns and heat transfer govern prill cooling and quality. Uniform airflow reduces turbulence and fines generation (Mehrez et al., 2014)
Optimised air inlet structures significantly influence outlet prill temperature and cooling behaviour (IJRASET, 2023).
Operational studies highlight that improper ventilation or asymmetric airflow elevate fine dust, confirming dust is often a mechanical/airflow issue, not just product quality (IJERT, 2021).
Ibrar Jamil from Fatima Fertilizer Company Ltd in Pakistan kicks off this round table discussion: Dust at the tower outlet is often blamed on melt quality, droplet formation, or crushing strength. But could airflow symmetry at the base be the bigger contributor? I’d love to hear from prilling tower operators, fertilizer process engineers, tower designers and troubleshooting specialists. Have you observed similar airflow-related effects? How do your facilities manage wind interaction and inlet airflow symmetry? Let’s share insights and best practices globally.
Kamran Ahmed of Todini Costruzioni Generali S.p.A. in Pakistan replies: From my time working in the Urea section (01 area) of FFC many years ago, I believe there are several factors that affect the quality of urea prills. These include urea melt temperature,which at the time was around 140°C, prill bucket size, typically 2.0 to 2.2 mm depending on operating conditions, bucket rpm, moisture content in the melt, air humidity, counter-current air temperature, wind direction, and uniform air distribution from all directions to avoid channeling and lump formation. Fines reduction also depends on carefully controlled operating conditions.
Beyond advanced design software, troubleshooting tools and especially APC can be extremely valuable, as they allow data collected over time to be used in simulated optimisation by adjusting key parameters to improve not only production but also the required product quality. At the same time, nothing can replace human field experience, including the day-to-day observations and practical lessons gained over years of shift work and operational exposure.
When this experience is combined with a fresh perspective from younger engineers, along with a three-dimensional way of thinking, it can lead to new ideas and improved focus, supported by software results and guided by the deep expertise of senior colleagues. In this way, the best combinations can be developed to address both existing and emerging issues.
Ibrar responds: Exactly, you have highlighted the critical parameters that affect prill quality, size, and dust generation. Currently, conventional operator experience and traditional process engineering mainly focus on melt temperature, moisture content, prilling bucket design, and mesh size, along with regular cleaning cycles. However, symmetrical airflow distribution at the tower inlet in natural draft prilling towers is often overlooked.
In my introduction, I tried to diagnose this factor and invite industry experts to discuss whether uniform airflow could significantly influence dust generation and prill quality. If this diagnosis is correct, it represents low-hanging fruit for operators: minor adjustments or trials in airflow could improve crystallisation, reduce fines, and optimise prill quality without affecting upstream process conditions. While we cannot control environmental factors like humidity or wind, better airflow management and prill cooling can enhance product consistency, reduce dust, and improve handling, which directly impacts customer satisfaction and market competitiveness.
In Pakistan, FFC already produces some of the highest-quality urea prills with minimal fines. Competitors like Fatima and Engro are striving to match this, and farmers increasingly demand top-quality, zero-dust prills to avoid crop damage during application. Looking ahead, as urea products diversify (neem-coated, zinc-coated, ureasecoated), airflow optimisation in prilling towers will remain a critical lever for quality, efficiency, and market differentiation.
Kamran comes back: I have not seen your prill tower system, but I do recall a market-wide comparison of bags in which FFC showed lower biuret and lower moisture, with a smaller prill size and fewer fines. However, farmers preferred the larger 2.2 mm prills which had a higher moisture content.
Since all three fertilizer plants are located in close proximity, wind direction and humidity are likely not materially different. However, the design of the intake air louvers and airflow control may be contributing factors. The height and diameter of the prill tower may also play an important role: greater tower height can reduce urea temperature and, in turn, lower fines. Intake air velocity, contact time, and the tower cross-sectional area will also affect fines generation and overall urea quality.
Intake air temperature and humidity can be managed through properly designed cooling coils, similar to the approach Turbomach GT in Switzerland used for Nishat Faisalabad, where chilled water coils at the air inlet were installed to reduce inlet air temperature and increase net power output, with a reported payback period of 2.5 years. The air velocity, available cross-sectional area, and contact time should be verified against the design specification sheet. If tower height is the limiting factor, then lowering the temperature could reduce dust and fines even without changing the tower height, potentially allowing for reduced airflow as well.
Has a variable control air intake system been installed?
Ibrar replies: Currently, airflow control is achieved primarily through damper adjustment and natural draft effects rather than a fully variable intake system. That is why I believe airflow symmetry and inlet area management are particularly important.
Kamran comes back again: I totally agree with you. Fully variable automated intake air system is a must and must be integrated with APC.
Tauseef Basit of Fatima Fertilizers Company Ltd in Pakistan joins the discussion: If top outlet louvers are blocked due to urea deposits can restrict air flow.
Ibrar replies: Thank you Tauseef, that’s a very important point. Restricted outlet louvers due to deposits could indeed affect the natural draft and reduce cooling efficiency. In our tower the outlet is an annular opening, and I haven’t personally seen any routine cleaning practice for that area. I’d be very interested to learn how other plants inspect or maintain the outlet stack/annulus space.
Aslam Syed, Ex Plant Manager Exterran Pakistan shares his valuable experiences: Air flow control is more effective at the bottom of the tower.
Ibrar comes back with more insights: One additional aspect worth considering in this discussion is the product temperature at tower discharge. In many operating environments, even when chemical specifications and prill size are within limits, elevated outlet temperatures can affect downstream performance, particularly in terms of stabilisation, coating effectiveness, and handling-related fines. Based on industry experience, prill temperatures are often maintained within a broad range – typically around 45–50°C under moderate ambient conditions – although this can vary depending on tower design, airflow efficiency, and seasonal factors. In that sense, temperature can serve as a useful secondary indicator of overall cooling performance rather than a standalone quality parameter.
It would be interesting to understand how different plants define and manage optimal temperature windows across varying climates and operating conditions.
I have seen some truly valuable insights shared here by industry experts and engineers. One clear takeaway from these exchanges is that dust and fines generation in prilling towers is rarely driven by a single factor. Airflow distribution, tower condition, droplet formation, melt quality, ambient conditions, and equipment health can all interact in complex ways.
Prilling towers are fascinating systems, where even small changes in airflow, heat transfer, or mechanical condition can sometimes have a disproportionate impact on prill cooling and dust formation. It is always interesting to learn how different plants approach troubleshooting and maintaining stability in these systems. Thank you to everyone for contributing to the discussion.
Leadership that is humble enough to listen, and teams willing to question, are what drive meaningful innovation and sustained growth. It is not always easy, but this is where real operational impact happens. Engineering progress has always been a collective effort, built on a combination of field experience, fresh perspectives, and respectful technical dialogue.
Beyond the technical discussion on prilling towers, a broader lesson from complex operations is that progress rarely comes from routine alone. Improvements often emerge when teams are willing to re-examine historical assumptions, share observations across functions, and discuss evidence openly.
